Showing papers in "Behavioral Ecology in 2010"

Dying young and living fast: Variation in life history across English neighborhoods.

Abstract: Where the expected reproductive life span is short, theory predicts that individuals should follow a ‘‘fast’’ life-history strategy ofearly reproduction, reduced investment in each offspring, and high reproductive rate. I apply this prediction to differentneighborhood environments in contemporary England. There are substantial differences in the expectation of healthy lifebetween the most deprived and most affluent neighborhoods. Using data from the Millennium Cohort Study (n ¼ 8660 families),I show that in deprived neighborhoods compared with affluent ones, age at first birth is younger, birthweights are lower, andbreastfeeding duration is shorter. There is also indirect evidence that reproductive rates are higher. Coresidence of a father figureis less common, and contact with maternal grandmothers is less frequent, though grandmaternal contact shows a curvilinearrelationship with neighborhood quality. Children from deprived neighborhoods perform less well on a verbal cognitive assess-ment at age 5 years, and this deficit is partly mediated by parental age and investment variables. I suggest that fast life history isa comprehensible response, produced through phenotypic plasticity, to the ecological context of poverty, but one that entailsspecific costs to children. Key words: birthweight, breastfeeding, grandmothering, humans, life-history theory, parental invest-ment, reproductive strategies. [Behav Ecol]

Self-organized aerial displays of thousands of starlings: a model

Abstract: Through combining theoretical models and empirical data, complexity science has increased our understanding of social behavior of animals, in particular of social insects, primates, and fish. What are missing are studies of collective behavior of huge swarms of birds. Recently detailed empirical data have been collected of the swarming maneuvers of large flocks of thousands of starlings (Sturnus vulgaris) at their communal sleeping site (roost). Their flocking maneuvers are of dazzling complexity in their changes in density and flock shape, but the processes underlying them are still a mystery. Recent models show that flocking may arise by self-organization from rules of co-ordination with nearby neighbors, but patterns in these models come nowhere near the complexity of those of the real starlings. The question of this paper, therefore, is whether such complex patterns can emerge by self-organization. In our computer model, called StarDisplay, we combine the usual rules of co-ordination based on separation, attraction, and alignment with specifics of starling behavior: 1) simplified aerodynamics of flight, especially rolling during turning, 2) movement above a ‘‘roosting area’’ (sleeping site), and 3) the low fixed number of interaction neighbors (i.e., the topological range). Our model generates patterns that resemble remarkably not only qualitative but also quantitative empirical data collected in Rome through video recordings and position measurements by stereo photography. Our results provide new insights into the mechanisms underlying complex flocking maneuvers of starlings and other birds.

Evolutionary forces favoring intragroup coalitions among spotted hyenas and other animals

Abstract: Coalitionary support in agonistic interactions represents cooperation because intervening in a fight is potentially costly to the donor of support but benefits the recipient. Here, we first review the characteristics of, and evolutionary forces favoring, intragroup coalitions in 49 species and find that patterns of intragroup coalition formation are remarkably similar between primates and nonprimates. We then test hypotheses suggesting kin selection, reciprocal altruism, and direct benefits as adaptive explanations for coalitionary interventions among adult female spotted hyenas (Crocuta crocuta) belonging to a large social group in Kenya. As predicted by kin selection theory, females supported close kin most often, and the density (connectedness) of cooperation networks increased with genetic relatedness. Nevertheless, kinship failed to protect females from coalitionary attacks. We found no evidence of enduring alliances based on reciprocal support among unrelated adult females. Instead, donors generally minimized costs to themselves, intervening most often during low-intensity fights and when feeding opportunities were unavailable. Females also gained direct benefits from directing coalitionary attacks toward subordinates. Finally, females monitored the number of dominant bystanders in the "audience" at fights and modified their level of cooperation based on this knowledge. Overall, hyenas made flexible decisions regarding whether or not to intervene in fights, modifying their tendency to cooperate based on multiple types of information about their immediate social and ecological environments. Taken together, these findings indicate that the combined evolutionary forces of kin selection and direct benefits derived from reinforcing the status quo drive coalitionary interventions among adult female spotted hyenas. Copyright 2010, Oxford University Press.

Interspecific variation in fear responses predicts urbanization in birds

Abstract: Urbanization and domestication share features in terms of characters that are favored by selection. These include loss of fear of humans, reduced corticosterone levels, prolonged breeding seasons, and several others. Here, I test the hypothesis that urbanization results from differential colonization of urban areas by species with heterogeneous levels of fear in the ancestral rural populations, followed by a reduction in variance in fear responses with a subsequent increase in diversity of fear responses as urban populations become adapted to the urban environment. Using information on variance in flight initiation distances (FIDs) when approached by a human, I show that rural populations of birds characterized by short mean flight distances and large variances in flight distances differentially colonized urban areas. As a consequence of this urban invasion, urban populations lost variation in FID. The variance in FID was initially larger in rural than in urban populations but eventually became larger in urban populations with time since urbanization. This secondary increase in variance in FID of urban populations was associated with an increase in population density of urban populations, suggesting that as birds became adapted to urban areas, they secondarily gained variance in behavioral flexibility. Key words: flight initiation distance, invasion, personality, urbanization. [Behav Ecol]

Behavioral types as predictors of survival in Trinidadian guppies (Poecilia reticulata)

Abstract: Studies of the fitness consequences of behavioral types often focus on isolated behaviors and ignore potential across-context correlations that may affect fitness. This approach leads to heterogeneous results across studies because correlations themselves may be adaptive in populations under significant predation pressure. We quantified suites of behaviors in 4 different contexts and identified a consistent behavioral syndrome in a population of Trinidadian guppies (Poecilia reticulata). We then measured fitness effects of the correlated behaviors that made up this syndrome and found that more active, bold, and exploratory individuals survived longer when exposed to a predator. Behavioral syndromes may, therefore, be advantageous in populations under significant predation risk if an individual's behavior in the presence of a predator is an honest signal of escape abilities. Interestingly, we also found a significant effect of the individual cichlids (Aequidens pulcher) used as predators in our experiments. We suggest that future studies should test whether interactions between predator behavior and prey behavioral types maintain behavioral variation. Copyright 2010, Oxford University Press.

Dominance in relation to age, sex, and competitive contexts in a group of free-ranging domestic dogs

Abstract: Current knowledge about social behavior of free-ranging domestic dogs is scarce, and the possibility that they could form stable social groups has been highly debated. We investigated the existence of a social-dominance hierarchy in a free-ranging group of domestic dogs. We quantified the pattern of dyadic exchange of a number of behaviors to examine to what extent each behavior fits a linear rank-order model. We distinguished among agonistic dominance, formal dominance, and competitive ability. The agonistic-dominance hierarchy in the study group shows significant and substantial linearity. As in random assortments of captive wolves, there is a prominent but nonexclusive male agonistic dominance in each age class. The agonistic rank-order correlates positively and significantly with age. Submissive‐affiliative behavior fulfills the criteria of formal submission signals; nevertheless, it was not observed among all dogs, and thus, it is not useful to order the dogs in a consistent linear rank. Agonistic-dominance relationships in the dog group remain stable across different competitive contexts and to the behaviors considered. Some individuals gain access to food prevailing over other dogs during competitions. Access to food resources is predicted reasonably well by agonistic rank order: High-ranking individuals have the priority of access. The findings of this research contradict the notion that free-ranging dogs are ‘‘asocial’’ animals and agree with other studies suggesting that long-term social bonds exist within free-ranging dog groups. Key words: age‐sex class relationships, Canis lupus familiaris, food competition, influence of competitive context, linear dominance hierarchy. [Behav Ecol]

Collective behavior in road crossing pedestrians: the role of social information

Abstract: Social information use is common in a wide range of group-living animals, notably in humans. We investigated social information use by pedestrians in a potentially dangerous scenario: at a road crossing. To judge a safe gap in traffic, pedestrians can use social information, such as the crossing behavior of others, and follow others across the road. We tested if pedestrians followed others in this scenario by analyzing pedestrian starting position and crossing order. First, we found that neighbors of a crossing pedestrian tended to cross before other waiting pedestrians and that this tendency was significantly higher in observed pedestrians than in a null model: a simulation in which pedestrians did not follow each other. Also, by fitting the null model, we found that on average a person was 1.5-2.5 times more likely to cross if their neighbor had started to cross. Second, we found that males tended to follow others more than females. Third, we observed that some individuals started to cross and then returned to the roadside. These individuals were more frequently found in groups and tended to start to cross relatively later than other pedestrians. These observations suggest that some of these individuals made incorrect decisions about the timing of their crossing and that this was due to social information use. Finally, we propose that the relatively small benefit of a reduced waiting time came at the cost of an increased risk of injury, making the beneficial value of social information use questionable in this context.

Personal immunity versus social immunity

Abstract: It is well known that organisms defend their fitness against attack from parasites and pathogens by mounting a personal immune response. However, there is increasing evidence that organisms from diverse taxa also exhibit immune responses for the purpose of protecting other individuals as well as themselves. We argue that any type of immunity that has fitness consequences for both the challenged individual and one or more recipients should be referred to as ‘social immunity’. We show that social immune systems are a widespread yet relatively neglected component of immunity, ideal for the study of social evolution. Whereas personal immune systems protect lifespan, social immune systems effectively defend the fecundity component of fitness, commonly protecting offspring or reproductive kin. We suggest that there are likely to be close links between life history and the extent of investment in each form of immunity. Furthermore, trade-offs between social and personal immunity may explain individual variation in personal immune responses, including sex-specific immune defences.

Agamas exhibit behavioral syndromes: bolder males bask and feed more but may suffer higher predation

Abstract: According to basic evoludonary theory, individuals within a population should adapt their behavior in response to their current physical and social environment. However, there is now evidence from a diverse range of taxa that behavior is instead constrained by individuals' broad behavioral syndromes or personalities. Bold individuals are generally shown to take greater risks than shy individuals. Theory suggests that there should be fitness trade-offs associated with personalities. We aimed to answer the following 3 questions using the Namibian rock agama (Agama planiceps) as a study species: 1) Is the boldness of individual male agamas repeatable? 2) Do male agamas show a behavioral syndrome? and 3) Are there any possible fitness trade-offs associated with individual behavioral syndromes. We measured boldness of 30 male agamas by quantifying flight initiation distance. We found that individual agama behavior was significantly consistent through time. Also, bolder males were found to spend significantly more time basking and moving throughout their home ranges in sight of predators, indicating a behavioral syndrome in this species. Bolder males also had larger home ranges and fed more than shyer males but suffered higher levels of tail loss, possibly clue to predation. The rates of visual signaling in the form of assertion displays of bolder and shyer males did not differ. We suggest that males of this species show a significant behavioral syndrome that may lead to fitness trade-offs. Key words: Agama planiceps, behavioral syndrome, boldness, individual variation, personality. [Behav Ecol 21:655-661 (2010)]

Flush early and avoid the rush: a general rule of antipredator behavior?

Abstract: Well-understood subjects should lead to the development of general hypotheses, rules, and ultimately, laws. Unlike the fields of physics, which has many laws (e.g., gravity, thermodynamics, etc.), ecology, which has a series of biogeographic rules (e.g., Allen’s, Bergmann’s, Cope’s, Gloger’s, Rapoport’s rules), and evolution, which has a grand theory (evolution by natural selection), behavioral ecology lacks such generalizations. There is a great deal of controversy about whether fields other than physics can or should have laws, and this has led to a spirited debate in the ecological literature (e.g., Lawton 1999; Mitchell 2000; Murray 2000; Jørgensen 2002; Colyvan and Ginzburg 2003; Lange 2005). I would like to acknowledge this controversy but note (that with some exceptions) behavioral ecologists often focus on documenting and understanding behavioral variation rather than summarizing it into formal hypotheses or rules. Some notable exceptions may include Hamilton’s rule (Hamilton 1964a, 1964b) and Trivers’ various hypotheses (Trivers 1971, 1974; Trivers and Willard 1973). I also acknowledge that some may consider these too restrictive and not having the precise predictive ability of what we would want a generalization to contain. Indeed, one might argue that a lesson from behavioral ecology is that all individuals face trade-offs, but this itself does not allow us to predict behavior in the way that a formal rule would. Rules, however specific, allow us to predict behavior. In this forumpiece, I would like to propose a hypothesis that, with time, may rise to a rule. The hypothesis is that ‘‘animals will flee approaching predators soon after they detect and identify them as a threat to reduce or minimize ongoing attentional costs of monitoring the approaching predators.’’ The implication of this ‘‘flush early and avoid the rush hypothesis’’ is that species that first detect threats at a greater distance (e.g., because of body size, eye size, habitat visibility, etc.) will be more distracted by approaching threats than those who are unable to detect them until they are closer. This differential distraction has implications for coexistence with both predators and humans. I pose this hypothesis in the spirit that by trying to make broad generalizations, and testing them, the field of behavioral ecology will advance well beyond the great conceptual advances made in the 1970s and 1980s (Birkhead and Monaghan 2010). Advances will range from identifying the conditions (i.e., state space, species, etc.) under which broad generalizations are possible, to perhaps generating truly universal rules. Animals may perceive humans as predators (Frid and Dill 2002), and this simple observation has allowed the systematic study of antipredator behavior across many taxa. When a human walks toward an animal, species that use flight to escape their predators will inevitably move away. This response is called ‘‘flight initiation distance’’ or, in the wildlife management literature, ‘‘flush distance’’ (Stankowich and Blumstein 2005). In many cases, prior to flight, the prey will orient or otherwise look at the approaching threat. This response is called ‘‘alert distance’’ (Blumstein et al. 2005). The distance from which a human begins approaching an individual is called ‘‘starting distance’’ (Blumstein 2003). In many cases, starting distance is used as a proxy for alert distance because it is often difficult to accurately identify when an animal becomes alert to an approaching threat. Indeed in some cases, animals may flush as soon as they become aware of an approaching threat (e.g., Stankowich and Coss 2006). The field will advance as we apply more sensitive assays to identify detection (e.g., cardiac or other ‘‘cryptic’’ autonomic nervous system responses—Blumstein and Bouskila 1996), so that we can better study the decision to flee. Flight initiation distance has been extensively reviewed (Stankowich and Blumstein 2005) and modeled (Ydenberg and Dill 1986; Blumstein 2003; Cooper and Frederick 2007). Studying factors that influence both flight initiation distance and alert distance has led to both theoretical and applied insights. Although animals may dynamically modify their alert and flight initiation distances to manage their response to threats, there is a species-specific component to these antipredator responses as well (Blumstein et al. 2003), and it is possible to study the evolution of both alert distance and flight initiation distance (Blumstein et al. 2005; Blumstein 2006). A general observation that emerges from studies of birds, mammals, and some lizards is that there is a statistical relationship between the distance the human starts walking toward the animal and both the alert distance and the flight initiation distance (Blumstein 2003; Blumstein et al. 2005; Cooper 2005; Stankowich and Coss 2006; Cooper et al. 2009) and that there is a statistical relationship between the alert distance and the flight initiation distance (Cárdenas et al. 2005). The relationship between starting distance and flight initiation distance is not always present (Cooper 2005), may in some circumstances be an artifact (Cooper 2008), and may be modified by other risk factors (Cooper et al. 2009), but it is nevertheless very common. The relationship between alert distance (which can’t always be properly measured, and thus starting distance is sometimes used as a proxy) and flight initiation distance seems more robust. Indeed this observation extends beyond vertebrates. While recently studying a terrestrial hermit crab (Coenobita clypeatus), I noticed that the distance a hermit crab first responded to an approaching human was highly correlated with the distance at which it withdrew into its shell and hid (r = 0.916, P , 0.0001, n = 60). This hiding initiation distance (Chan et al. 2010) is an analog to flight initiation distance in a species that hides, rather than flees, from approaching humans. This general relationship—seen in birds, mammals, lizards, and now an invertebrate—is not simply driven by the fact that animals that are first approached at a very close distance must orient and flush at a closer distance than those who are first approached at a farther distance. Indeed, the range of starting distances in well-studied species is broad (Blumstein et al. 2005). And, even if one starts approaching an animal from a relatively close distance, they often fail to respond for a while. If anything, there may be a bias in the other direction in that

Exposure to rivals and plastic responses to sperm competition in Drosophila melanogaster

Abstract: Responses by males to the level of sperm competition have been documented across a wide range of taxa. Recent work in Drosophila melanogaster shows that males respond adaptively to the presence of other males by making facultative adjustments to mating duration, resulting in increased transfer of ejaculate proteins, direct effects on postmating responses in females, and, ultimately, increased male competitive reproductive success. Here, we investigated how males detect the presence of rival males. We tested the effect of the length of male-to-male exposure, male age at first exposure, time since initial exposure to rivals and density. We found that the longer the males were exposed to rivals prior to mating (from 0 to 101 h of exposure), the longer their subsequent mating duration. There was no detectable effect, however, of increasing the number of rivals above 1. Increasing the density (hence encounter rate) in which males were kept had no effect on a male's response to rivals and there was also no evidence that responses to rivals could be evoked by a brief (2 h) time window of exposure to males at various times prior to mating. The age at which males were first exposed to other males did not affect their ability to respond to rivals. Taken together, our findings show that it is the absolute length of exposure to rivals and not the number of rivals that is critical in determining male plastic responses to the potential level of sperm competition in D. melanogaster.

Songbird chemosignals: volatile compounds in preen gland secretions vary among individuals, sexes, and populations.

Abstract: Chemical signaling has been documented in many animals, but its potential importance in avian species, particularly songbirds, has received far less attention. We tested whether volatile compounds in the preen oil of a songbird (Junco hyemalis) contain reliable information about individual identity, sex, or population of origin by repeated sampling from captive male and female juncos originating from 2 recently diverged junco populations in southern California. One of the populations recently colonized an urban environment; the other resides in a species-typical montane environment. The birds were field-caught as juveniles, housed under identical conditions, and fed the same diet for 10 months prior to sampling. We used capillary gas chromatography‐mass spectrometry to quantify the relative abundance of 19 volatile compounds previously shown to vary seasonally in this species. We found individual repeatability as well as significant sex and population differences in volatile profiles. The persistence of population differences in a common environment suggests that preen oil chemistry likely has a genetic basis and may thus evolve rapidly in response to environmental change. These finding suggest that songbird preen oil odors have the potential to function as chemosignals associated with mate recognition or reproductive isolation. Key words: birds, chemical communication, Junco hyemalis, olfaction, pheromones. [Behav Ecol]

Differential allocation and compensation: who deserves the silver spoon?

Abstract: It is well known that being given ‘‘silver spoons’’ can be beneficial to offspring, in the sense that conditions experienced early in life can have long-lasting consequences (Lindstrom 1999; Qvarnstrom and Price 2001). Traits expressed by parents often form a large part of the offspring environment. Life-history theory predicts that parents should adjust their current reproductive investment according to both the expected pay-off from the current attempt and the expected future reproductive events (Stearns 1992). Thus, for selection to favor increased investment, the cost of decreased survival or future reproductive success for the parent should be compensated by higher survival or reproductive success of the current offspring. Because offspring of diploid species have a mother as well as a father, a parent of either sex might benefit by adjusting its investment based on the phenotypic or genetic traits of its mate. Since Burley’s (1986) first formulation, this idea has been called differential allocation (DA). Burley’s early studies focused on a particular aspect of the interaction between costs and benefits experienced by the mother and the father: In her hypothesis that an individual could benefit by increasing effort when mated to an attractive member of the opposite sex because this might enhance the ability to maintain the pairbond with this mate now or in future breeding attempts (Burley 1986, 1988). The discrepancy between early work that focused on pair-bond maintenance and current much broader definitions is probably a healthy sign of progress in a field. Even so, in this paper, we would like to point out that the field is still plagued by differences between researchers in what they consider DA and in which direction they predict allocation of resources to vary. Particularly, the ‘‘compensation hypothesis’’ has recently been proposed as a mechanism that appears to make opposite predictions to the DA hypothesis, but as we shall show, there is much clarification needed before we can reach a stage where the core ideas are crisply stated and the relationships between various hypotheses are properly evaluated. We will show that similar clarifying work needs to occur in the development of theory as well as in empirical studies.

Color signal information content and the eye of the beholder: a case study in the rhesus macaque

Abstract: Animal coloration has provided many classical examples of both natural and sexual selection. Methods to study color signals range from human assessment to models of receiver vision, with objective measurements commonly involving spectrometry or digital photography. However, signal assessment by a receiver is not objective but linked to receiver perception. Here, we use standardized digital photographs of female rhesus macaque (Macaca mulatta) face and hindquarter regions, combined with estimates of the timing of the female fertile phase, to assess how color varies with respect to this timing. We compare objective color measures (camera sensor responses) with models of rhesus vision (retinal receptor stimulation and visual discriminability). Due to differences in spectral separation between camera sensors and rhesus receptors, camera measures overestimated color variation and underestimated luminance variation compared with rhesus macaques. Consequently, objective digital camera measurements can produce statistically significant relationships that are probably undetectable to rhesus macaques, and hence biologically irrelevant, while missing variation in the measure that may be relevant. Discrimination modeling provided results that were most meaningful (as they were directly related to receiver perception) and were easiest to relate to underlying physiology. Further, this gave new insight into the function of such signals, revealing perceptually salient signal luminance changes outside of the fertile phase that could potentially enhance paternity confusion. Our study demonstrates how, even for species with similar visual systems to humans, models of vision may provide more accurate and meaningful information on the form and function of visual signals than objective color measures do.

Alarm calls of tufted titmice convey information about predator size and threat

Abstract: Many birds utter alarm calls when they encounter predators, and previous work has revealed that variation in the characteristics of the alarm, or ‘‘chick-a-dee,’’ calls of black-capped (Poecile atricapilla) and Carolina (P. carolinensis) chickadees conveys information about predator size and threat. Little is known, however, about possible information conveyed by the similar ‘‘chick-a-dee’’ alarm call of tufted titmice (Baeolophus bicolor). During the winters of 2008 and 2009, free-ranging flocks (N ¼ 8) of tufted titmice were presented with models of several species of raptors that varied in size, and titmice responses were monitored. Smaller, higher threat predators (e.g., eastern screech-owl, Megascops asio) elicited longer mobbing bouts and alarm calls with more notes (D-notes) than larger lower threat predators (e.g., red-tailed hawk, Buteo jamaicensis). During playback experiments, titmice took longer to return to feeding after playbacks of alarm calls given in response to a small owl than to playbacks given in response to a large hawk or a robin (control). Like chickadees, titmice appear to utter alarm calls that convey information about predator size and threat. Titmice, however, appear to cue in on the total number of D-notes given per unit time instead of the number of D-notes per alarm call. The broadband alarm calls of titmice, containing D-notes uttered in an unpredictable manner that may make it difficult for predators to determine the number of titmice calling, may be particularly well suited for deterring predators.

Unpredictability in food supply during early life influences boldness in fish

Abstract: Behavioral variation has been documented both between and within populations in a variety of traits. Many of these behavioral traits are phenotypically plastic and are conditional on the early environment an animal experiences, yet despite this the role of the environment in generating variation in boldness is not well understood. Here we investigate the importance of early and recent experience of temporal unpredictability in food supply on the behavior of a species of freshwater fish, the Trinidadian guppy Poecilia mticulata. We predict that individuals that experience temporally unpredictable food supplies will engage in more risky behavior than those experiencing a Predictable food supply and find evidence to support this. Fish with early experience of unpredictable environments are generally bolder and more exploratory than fish reared in predictable environments, exploring a significantly greater proportion of a novel maze and spending less time in a refuge during the trial. Individuals with early experience of unpredictability also spent significantly less time associating with conspecifics in a shoaling tendency assay, again suggesting that they are bolder than guppies reared in predictable conditions. These findings suggest that early experience is important in shaping exploratory anti shoaling behavior in this species and that unpredictability in early life can influence boldness in guppies. (Less)

Taller men are less sensitive to cues of dominance in other men

Abstract: Male dominance rank, physical strength, indices of reproductive success, and indices of reproductive potential are correlated with masculine characteristics in many animal species, including humans. Accordingly, men generally perceive masculinized versions of men’s faces and voices to be more dominant than feminized versions. Less dominant men incur greater costs when they incorrectly perceive the dominance of rivals. Consequently, it may be adaptive for less dominant men to be particularly sensitive to cues of dominance in other men. Because height is a reliable index of men’s dominance, we investigated the relationship between own height and men’s sensitivity to masculine characteristics when judging the dominance of other men’s faces and voices. Although men generally perceived masculinized faces and voices to be more dominant than feminized versions, this effect of masculinity on dominance perceptions was significantly greater among shorter men than among taller men. These findings suggest that differences among men in the potential costs of incorrectly perceiving the dominance of rivals have shaped systematic variation in men’s perceptions of the dominance of potential rivals. Key words: dominance, sexual dimorphism. [Behav Ecol]

Sex-specific behavior by a monomorphic seabird represents risk partitioning

Abstract: The presence of sex-stereotyped behavior in monomorphic animals, where there are no sexual differences in form to account for sexual differences in function, is often attributed to intraspecific competition or to differential parental investment. The possibility that the use of different behavioral strategies by each parent may increase reproductive success for both partners through risk partitioning is seldom considered. We studied thick-billed murres (Uria lomvia), where the male exclusively feeds the offspring during the late chick rearing. During the period of biparental care, males fed on ‘‘risk-averse’’ prey (consistent across time and space; unitized risk ¼ 0.29), whereas females fed on ‘‘risk-prone’’ prey (risk ¼ 0.59). Males fed at night at 1 colony, during the day at 2 colonies, and there was no pattern at another colony. We suggest that these differences reflect the availability of risk-prone prey. Modeling suggested that mixed-risk pairs had higher success than ‘‘risky’’ or ‘‘riskless’’ pairs. Males accumulated reserves and reduced chick provisioning just prior to fledging. Thus, sex-specific patterns at 1 period (male-only care during postfledging) may have led to sex-specific patterns at earlier periods through the need for specialization in foraging habits and risk. We propose that risk partitioning may contribute to the prevalence of sex-specific behaviors in monomorphic animals and that patterns are likely context specific rather than species specific. Key words: risk aversion, sex-specific behavior, thick-billed murre, Uria lomvia. [Behav Ecol]

Information use and density-dependent emigration in an agrobiont spider

Abstract: Density of conspecifics is considered as one of the main conditions affecting dispersal behavior and leading to a stabilization of population dynamics. Density-dependent dispersal can be induced by local competition at different phases during development and/or by density-related sources of social information. Here, we assessed the importance of population density on emigration rates and the degree to which the presence of silk threads at dispersal takeoff locations affects immediate dispersal decision making in the spider Erigone atra. By quantifying behaviors in wind tunnels under standardized laboratory conditions, silk-assisted long- and short-distance dispersal is quantified before the actual onset of the dispersal event. Increased densities during juvenile development only affected short-distance dispersal behavior. In females, short-distance dispersal increased with the female density experienced during development, whereas responses in males increased under combined high male/low female-experienced densities. Elevated densities at the onset of dispersal led to a general increase of predispersal behaviors. The presence of silk threads at takeoff platforms similarly induced an increase of dispersal displays, with specifically an increase in long-distance dispersal in both sexes. Our results demonstrate that the spider E. atra uses information related to density during development, most probably to avoid competition by performing short-distance dispersal. In contrast, density-related cues at the time of dispersal (i.e., increased densities and the presence of silk threads) increase general dispersal activities and long-distance ballooning events. Short- and long-distance dispersal strategies are consequently guided by differential density-related information use. Key words: araneae, density, dispersal, information, silk. [Behav Ecol]

Tactical allocation of effort among multiple signals in sage grouse: an experiment with a robotic female

Abstract: Males in many species have complex, multicomponent sexual signals, and there may be trade-offs between different signal components. By adjusting their signaling behaviors, males may be able to produce more attractive courtship displays in the face of these trade-offs, but this possibility has rarely been tested. In this study, we examined adaptive adjustment of display behaviors during courtship in a lek-breeding bird, the greater sage grouse (Centrocercus urophasianus). We measured the potential trade-off between display quantity (display rate) and quality (a temporal feature of displays) in a wild population of sage grouse using controlled approaches of a robotic female to experimentally induce changes in male display rate. We found that males who are more successful in mating can increase quantity without a decline in quality, with only unsuccessful males expressing an apparent trade-off. Male mating success was also positively correlated with responsiveness to changes in receiver distance, suggesting that successful males may avoid a trade-off by tactically adjusting their display rate—saving energy by displaying at low levels when females are farther away and at higher levels as females approach. Alternative explanations for this differential response to female proximity are discussed. Our results suggest that to be successful, males may need both the ability to produce attractive signals and the ability to effectively allocate their display effort by responding to female behaviors. Key words: acoustic localization, communication, greater sage grouse, multiple traits, plasticity, sexual selection, trade-off. [Behav Ecol]

Small folivorous primate groups exhibit behavioral and physiological effects of food scarcity

Abstract: The influence of diet and food distribution on the socioecology of group-living species has long been debated, particularly for primates. It has typically been assumed that folivorous primates experience relatively little feeding competition due to the abundant, widespread nature of their food, freeing them to form large groups in response to predation, to disperse with relative ease, and to have egalitarian female social relationships. Recent studies, however, have come to different conclusions about the extent to which folivorous primates are limited by food and experience food competition and how these factors affect folivore socioecology. To better understand the selective pressures that diet places on folivores, we investigated how 2 small highly folivorous groups of colobus monkeys (Colobus guereza) in Kibale National Park, Uganda, responded behaviorally and physiologically to a steep reduction in availability of their most important foods. The monkeys decreased their reliance on their 2 most frequently eaten food species and increased their daily path length, number of feeding patches visited/day, size of individual feeding areas, percentage of time spent feeding, and dietary diversity. They also showed evidence of physiological costs, in that lactating females' urinary C-peptide levels (i.e., insulin production) declined as top foods became scarce, and parasite loads slightly, but significantly, increased in 2 of 3 adult females examined. These results suggest that highly folivorous primates, even in very small groups, may experience behavioral and physiological effects of food limitation, within-group scramble competition for food, and possibly substantial selective pressures during periods of food scarcity. Copyright 2009, Oxford University Press.

Induction of settlement in crab megalopae by ambient underwater reef sound

Abstract: The larvae of a number of crab species have been found to orientate and swim toward ambient underwater sound emanating from coastal settlement habitats. This current study examined whether ambient underwater sound also has the potential to trigger settlement responses in crab larvae. The effect of exposure to reef sound on the settlement behavior and time to metamorphosis (TTM) was examined in the megalopae of 5 common crab species, 3 from temperate waters and 2 from tropical waters. The megalopae of all 5 crab species showed marked changes in swimming behavior and a significant decrease in TTM when exposed to replayed ambient underwater reef sound compared with a silent (control) treatment. Megalopae exposed to sound decreased swimming activity earlier and displayed crawling behavior that was a precursor to both settlement and metamorphosis. Sound exposure decreased the median TTM by 33 h in Hemigrapsus sexdentatus and by 75 h in Grapsidae sp. 2. The consistent results among all species examined indicate that ambient underwater sound is likely to be an important settlement cue for the megalopae of many crab species. The wider ecological significance of acoustic settlement cues in crab larvae, in relation to other settlement cues and processes, now needs to be determined. Key words: crab, megalopae, metamorphosis, settlement cue, underwater sound, settlement cue. [Behav Ecol]

Sex and social networking: the influence of male presence on social structure of female shark groups

Abstract: Marine predators such as sharks often form single-sex aggregations as part of their diel behavioral cycle. Such aggregations are potentially driven by contrasting reproductive and behavioral strategies between the sexes, leading to distinct sexual segregation. There is, however, no experimental evidence that such predator aggregations are governed by intrinsic social systems, demonstrating long-term temporal stability. Social network structure, temporal stability, and activity profiles were analyzed to examine the impact of introduced males on social structure of 4 captive groups of female catshark (Scyliorhinus canicula). Aside from foraging and habitat selection pressures, we question whether individual female sharks manage additional social demands that potentially influence activity budget and susceptibility to male harassment. Shark groups demonstrated marked differences in their tendency to aggregate in a unisex environment and concomitantly showed significant differences in social response to male presence. Dichotomy in social structure of the 4 groups revealed that well-connected individuals demonstrated greater resilience to male disturbance. Socially isolated females experienced greater levels of partner exchange, reduced temporal stability, and increased activity levels in the presence of males and control females. These results are the first to demonstrate the disruptive influence that male and novel females have on weak female shark social interactions; conversely, aggregations demonstrating relatively strong temporally stable social bonds (at least by some individuals) are particularly resilient to potential male harassment and novel (female) intruders. We highlight that additional ‘social constraints’ may have disparate influence on individuals occupying different structural positions within a network and may thus impact individual fitness unequally. Key words: aggregation, behavior, fish, network analysis, sexual segregation, shark, social structure. [Behav Ecol]

Terminal investment and senescence in rhesus macaques (Macaca mulatta) on Cayo Santiago

Abstract: Long-lived iteroparous species often show aging-related changes in reproduction that may be explained by 2 non-mutually exclusive hypotheses. The terminal investment hypothesis predicts increased female reproductive effort toward the end of the life span, as individuals have little to gain by reserving effort for the future. The senescence hypothesis predicts decreased female reproductive output toward the end of the life span due to an age-related decline in body condition. Nonhuman primates are ideal organisms for testing these hypotheses, as they are long lived and produce altricial offspring heavily dependent on maternal investment. In this study, we integrated 50 years of continuous demographic records for the Cayo Santiago rhesus macaque (Macaca mulatta) population with new morphometric and behavioral data to test the senescence and terminal investment hypotheses. We examined relationships between maternal age and activity, mother and infant body condition, interbirth intervals, measures of behavioral investment in offspring, and offspring survival and fitness to test for age-associated declines in reproduction that would indicate senescence, and for age-associated increases in maternal effort that would indicate terminal investment. Compared with younger mothers, older mothers had lower body mass indices and were less active, had longer interbirth intervals, and spent more time in contact with infants, but had infants of lower masses and survival rates. Taken together, our results provide strong evidence for the occurrence of reproductive senescence in free-ranging female rhesus macaques but are also consistent with some of the predictions of the terminal investment hypothesis.

Evolutionary determinants of modular societies in colobines

Abstract: Modular societies are structurally characterized by nuclear one-male units (OMUs, or harems) embedded within larger relatively coherent social bands. Within the order Primates, modular societies are uncommon, found in only a few species, including humans. Asian colobines (Presbytini) principally form either unimale groups that forage independently and are often territorial, or modular associations, which range from tight bands composed of OMUs to loose neighborhoods of OMUs. A phylogenetic reconstruction of modularity in the Presbytini revealed that the single OMU pattern is probably the ancestral state while the modular pattern is derived. The selective forces favoring the evolution of modular societies have thus far been virtually unexplored. Although some ecological explanations cannot be ruled out at the moment due to lack of comparative and quantitative data, preliminary circumstantial evidence does not seem to support them. Instead, a social factor, bachelor threat, is consistent with many observations. This hypothesis argues that where the pressure from nonreproductive bachelor males is unusually high, OMUs aggregate as a means of decreasing the amount of harassment and the risk of takeovers and infanticide. A comparative test found an association between modular societies and bachelor threat, as proxied by sex ratio within social units. The concentration of modular systems in colobines may be due to their unusual ecology, which leads to unusually low intensity of scramble competition. Modular colobines rely more on nonlimiting ubiquitous resources than nonmodular ones and thus can afford to gather in bands. Moreover, by comparing the slopes of regressions between group size and daily travel distance for several groups of one modular and one nonmodular colobine, we found slopes in the nonmodular to be steeper by a factor 30, indicating that ecological constraints associated with scramble competition prevent higher level groupings in nonmodulars. Thus, modular sociality in Asian colobines may have arisen because both social benefits are substantial and ecological costs are relatively low.

A behavioral mechanism underlying ecological divergence in the malaria mosquito Anopheles gambiae

Abstract: Disruptive selection mediated by predation on aquatic immature stages has been proposed as a major force driving ecological divergence and fostering speciation between the M and S molecular forms of the African malaria mosquito, Anopheles gambiae. In the dry savannahs of West Africa where both molecular forms co-occur, the S form thrives in temporary pools filled with rainwater, whereas the M form preferentially breeds in permanent freshwater habitats where predator pressure is higher. Here, we explored the proximal mechanisms by which predation may contribute to habitat segregation between molecular forms using progeny of female mosquitoes captured in Burkina Faso. We show that the S form suffers higher predation rates than the M form when simultaneously exposed to the widespread predator, Anisops jaczewskii in an experimental arena. Furthermore, behavioral plasticity induced by exposure to the predator was observed in the M form, but not in the S form, and may partially explain its habitat use and ecological divergence from the S form. We discuss the role of adaptive phenotypic plasticity in allowing successful colonization of a new ecological niche by the M form and highlight further research areas that need to be addressed for a better understanding of the ultimate mechanisms underlying ecological speciation in this pest of major medical importance.

Determining interaction rules in animal swarms

Abstract: In this paper, we introduce a method for determining local interaction rules in animal swarms. The method is based on the assumption that the behavior of individuals in a swarm can be treated as a set of mechanistic rules. The principal idea behind the technique is to vary parameters that define a set of hypothetical interactions, as for example, a rule for aligning with neighbors. The parameter values are optimized so that the deviation between the observed movements in an animal swarm and the movements predicted by the assumed rule set is minimal. We demonstrate the method by reconstructing the interaction rules from the trajectories produced by a computer simulation.

Prey concealment: visual background complexity and prey contrast distribution

Abstract: A prey may achieve camouflage through background matching and through disruptive coloration. Background matching is based on visual similarity between the prey and its background, whereas disruptiv ...

Predator lethality, optimal escape behavior, and autotomy

Abstract: Flight initiation distance is the distance separating predator and prey when escape begins. The optimal flight initiation distance occurs where expected postencounter fitness is maximized, which depends on the prey's initial fitness, benefits obtainable by not fleeing, energetic escape costs, and expected fitness loss due to predation risk. In current optimal escape theory, prey die when contacted by a predator. We explore effects of variable lethality, L, the probability of being killed on contact. Optimal flight initiation distance increases as lethality increases, matching expectations that prey should not flee when contact entails no fitness loss but should be increasingly wary as expected fitness loss on contact increases. Addition of lethality improves the ability of optimal escape theory to predict effects of factors affecting escape ability. Autotomy, the voluntary shedding of tails or other expendable parts as a last-ditch defense to permit escape, provides an example. After autotomy, running speed decreases in many prey, lethality increases because autotomy cannot be used again until the lost part has regenerated, and ability to obtain benefits may decrease due to reduced social status and foraging ability. These changes favor longer flight initiation distance but lowered initial fitness after autotomy has the opposite effect. Optimal escape theory including a lethality term clarifies how autotomy may lead to increase or decrease in flight initiation distance depending on the balance of its multiple effects. Effects of additional factors that may alter multiple parameters of the model, including age, sex, reproductive condition, injury, disease, and parasitism are discussed. Copyright 2009, Oxford University Press.

Examination of prior contest experience and the retention of winner and loser effects

Abstract: In many animal taxa, prior contest experience affects future performance such that winning increases the chances of winning in the future (winner effect) and losing increases the chances of losing in the future (loser effect). It is, however, not clear whether this pattern typically arises from experience effects on actual or perceived fighting ability (or both). In this study, we looked at winner and loser effects in the jumping spider Phidippus clarus. We assigned winning or losing experience to spiders and tested them against opponents of similar fighting ability in subsequent contests at 1-, 2-, 5-, and 24-h intervals. We examined the strength of winner and loser effects, how long effects persist, as well as how experience affected perceived and actual fighting ability. Our results demonstrate that winner and loser effects are of approximately the same magnitude, although loser effects last longer than winner effects. Our results also demonstrate that previous experience alters actual fighting ability because both the assessment and escalation periods were affected by experience. We suggest that the retention time of experience effects depends on expected encounter rates as well as other behavioral and ecological factors. In systems with short breeding seasons and/or rapidly fluctuating populations, context-dependent retention of experience effects may allow males to track their status relative to the fluctuating fighting ability of local competitors without paying the costs necessary to recall or assess individual competitors.